How Can Culture Be Both Everywhere and Invisible?

This is one of the paradoxes of culture.  

One page 96 in The Practical Skeptic, I quote from Ralph Linton, a famous anthropologist.  He describes the paradox this way:

The last thing which a dweller in the deep sea would be likely to discover would be the water.

Or, as I like to paraphrase it: the last thing a fish would notice is the water.

Here's how Linton explained his point:

He would become conscious of its existence only if some accident brought him to the surface and introduced him to air.  [Likewise] Throughout most of history, [people have] been only vaguely conscious of the existence of culture and has owed even this consciousness to contrast between the customs of [their] own society and those of some other with which [they] happened to be brought into contact. [Ralph Linton, 1945] (pg 95 in The Practical Skeptic)

The fish don't see the water until their environment changes; likewise, humans didn't discover they had culture until they saw different ones!

And, I might add, it was the shock of this discovery that led Europeans to explore their own culture.